1. Field of the Invention
The present invention relates to a resin-coated carrier for an electrophotographic developer used in copying or printing machines, particularly for full color image formation and to an electrophotographic developer containing the carrier.
2. Description of the Related Art
A two-component developer used in electrophotography comprises a toner and a carrier. The carrier is mixed and agitated with the toner in a development box to give a desired quantity of charges to the toner and carries the charged toner particles onto an electrostatic latent image formed on a photoreceptor to form a toner image.
The carrier remains on the magnet roll and is returned to the development box, where it is again mixed and agitated with fresh toner particles for repeated use.
Therefore, the carrier is required to exhibit desired characteristics, especially charging properties for toner particles, constantly in any environment over its service life.
Conventional developers, however, undergo a so-called spent-toner phenomenon that toner particles adhere by fusion to the surface of the carrier particles due to the stress of friction and/or collision of carrier particles with each other or with the wall of a development box. The stress also causes a resin coat of carrier particles to separate and fall off the core. On account of these phenomena, the carrier characteristics such as charge quantity and carrier resistance vary, which can result in image deterioration (such as change in image density and fog) or toner scattering.
In order to prevent the deterioration of carrier characteristics, studies have been made on the resin for coating the surface of a carrier. Of various resins proposed to date acrylic resins and silicone resins have now been prevailing.
Acrylic resins exhibit good adhesion to a carrier core, and an acrylic resin-coated carrier has excellent charging ability, particularly for a negatively chargeable toner, and therefore, has been used widely. However, acrylic resin-coated carrier has a disadvantage of poor resistance against the spent-toner phenomenon. A silicone-coated carrier, on the other hand, has poor charging ability, although it is resistant to the spent-toner phenomenon owing to its low surface energy. Recently, an amino-containing silicone-coated carrier has been proposed (Japanese Patent Laid-Open No. 104522/95), in which the charging ability is improved to the level of acrylic resin-coated carriers by incorporating an amino group to the coating silicone resin.
Since a silicon coating layer is highly insulating, a silicone-coated carrier causes the developer to change (increase) the charge quantity during use, which results in variation of image characteristics. Change in image density with an increase of charge quantity during use is very problematical particularly in full color image formation having weight attached to gradation.
Acryl-modified silicone-coated carriers have been proposed as a carrier possessing both the merits of the acrylic resin-coated carrier and the silicone-coated carrier as disclosed in Japanese Patent Laid-Open Nos. 157751/80 and 234501/96. The coated carriers disclosed are excellent in both charging ability and charging stability and superior to the acryl resin-coated one in resistance against the spent-toner phenomenon.
However, the conventional acryl-modified silicone-coated carriers have poor fluidity. In particular, fluidity of those particles having an average particle size as small as 60 .mu.m or less are so poor that carrier transfer properties on the magnet roll during development are reduced to reduce the ability of developing the photoreceptor with a toner. The poor fluidity also causes non-uniform mixing with toner particles in the development box.
In regards to resistance to a spent phenomenon, the conventional acryl-modified silicone-coated carriers are, while superior to acrylic resin-coated carriers as mentioned above, still inferior to silicone-coated carriers, which is problematic particularly to carrier particles for full color machines which are brought into contact with toner particles more frequently. The aforementioned problems have not been settled as yet.